# [petsc-users] Using petsc for banded matrices and 2D finite differences

Jed Brown jedbrown at mcs.anl.gov
Tue Nov 15 14:59:54 CST 2011

```On Tue, Nov 15, 2011 at 14:49, Brandt Belson <bbelson at princeton.edu> wrote:

> Sorry, I was mistaken about the dt/Re coefficient. I use an implicit time
> step on the linear part of incompressible Navier-Stokes, so roughly
> discretizing du/dt = 1/Re * Lu with implicit Euler for simplicity gives:
>
> (u^{n+1} - u^n) / dt = 1/Re L u^{n+1}
> rearranging:
> (I - dt/Re L) u^{n+1} = u^n
>
>

Right. And you are treating the (nonlinear) convective term explicitly?

>
> For tri-diagonal matrices, I believe the direct solve is O(N), and similar
> methods exist for block tri-diagonal matrices. I know multigrid is also
> O(N), but I've heard it tends to be slower.
>

The "block tri-diagonal" matrices have to deal with fill. They are not O(N)
because the bandwidth b=sqrt(N), so you need b*N storage and b^2*N time (as
Jack said). At high Reynolds number and short time steps (high resolution),
your system is pretty well-conditioned because it is mostly identity. You
should be able to solve it very fast using iterative methods (perhaps with
a lightweight multigrid or other preconditioner).

>
> I think the bandwidth (b) of the matrices is small. For example for a
> 5-point stencil (center, left, right, below, above) the bandwidth is 5 and
> for a 9-point stencil it is 9.
>

No, if the neighbors in the x-direction are "close", then the neighbors in
the y-direction will be "far" (order n=sqrt(N) for an n*n grid).

>
> The other limiting factor of doing option 1, with the all-to-all, is the
> number of nodes is limited by nz. That might be ok, but I would like the
> code to scale to larger grids in the future.
>

Hence Jack's hybrid suggestion of using process blocks to handle chunks. As
usual with parallel programming, choose a distribution and write your code,
but as you write it, keep in mind that you will likely change the
distribution later (e.g. don't hard-code the bounds for "local"
computations).
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